The present invention relates to ferromagnetic thin-film structures exhibiting relatively large magnetoresistive characteristics that are used to couple digital signals from a source to an isolated receiver magnetically and, more particularly, to circuits used in such coupling and to monitor performance of such coupling structures.
Many kinds of electronic systems make use of magnetic devices including both digital systems, such as memories, and analog systems such as field sensors. Magnetometers and other magnetic field sensing devices are used extensively in many kinds of systems including magnetic disc memories and magnetic tape storage systems of various kinds. Such devices provide output signals representing the magnetic field sensed thereby in a variety of situations.
One use for such magnetic field sensors is the sensing of magnetic fields generated by electrical currents in a conductor as a basis for inferring the nature of such currents giving rise to these fields. While this has long been done for magnetic fields generated by substantial currents, such sensing becomes more difficult to accomplish in smaller ranges of currents that include relatively quite small currents. The need for sensing fields due to such currents arises, for instance, in situations where the currents generating the field to be measured are provided merely as a basis for conveying signal information rather than for transmitting substantial electrical energy.
Such a situation occurs in many medical systems, instrumentation systems and control systems where there is often a need to communicate signals to system portions over signal interconnections from an external source or from another portion of the system. Often, the conductors carrying signal currents for such purposes must be electrically isolated from the portion of the system containing the sensor arrangement provided for those signals to measure the resulting magnetic field. As an example, a long current loop carrying signal information in the loop current may, through lightning or static electricity discharges, become subject to having large voltage potentials relative to ground developed thereon. Such potentials must in many instances be kept from the signal sensing and receiving circuitry to avoid damage thereto even though that circuitry must still be able to capture the signal information contained in the loop current.
Signal isolators for these purposes are often preferably formed in monolithic integrated circuit chips for reasons of cost, convenience and system performance. In such an arrangement, one or more solid state magnetic field sensors are used to detect the magnetic fields provided by the currents containing the signals. One effective arrangement that has emerged in these circumstances for signal isolation in both hybrid integrated circuits and monolithic integrated circuits comprises an isolator input side input conductor, typically in some coiled electrical current conductor configuration, and an isolator receiver or output side magnetically based current sensor both supported on a substrate adjacent to and spaced apart from the input conductor so that they are electrically isolated from one another but with the current sensor positioned in those magnetic fields arising from any input currents appearing in the input side conductor. The receiver current sensor is often connected to an amplifier to thereby form a system operating as a current determiner. Such an isolator or determiner is an attractive device for these purposes in being both rapid in operation and economic low in cost, and has been disclosed in U.S. Pat. No. 5,831,426 to W. C. Black and T. M. Herrmann entitled “Magnetic Current Sensor”, and in U.S. Pat. No. 6,300,617 to J. M. Daughton, R. T. Fayfield, T. M. Hermann and J. F. Stokes entitled “Magnetic Digital Signal Coupler” both of which have been assigned to the same assignee as the present application and both of which are hereby incorporated herein by reference.
These current sensors are typically magnetoresistive effect based sensors. They are typically formed with an intermediate thin-film layer of a nonmagnetic separating material having two major surfaces on each of which an anisotropic ferromagnetic thin-film is positioned which has been found to lead to a “giant magnetoresistive effect” in the sensor if the thickness of the ferromagnetic thin-films and the conductive intermediate layer in such a “sandwich” structure have been made sufficiently small, or to a magnetically controlled tunnel diode for such small thicknesses in which there is instead an insulative intermediate layer.
In such monolithic integrated circuit based signal isolators, power dissipation can be reduced along with the risks of electromigration of the conductors therein, and so the reliability thereof can be increased, all by using a differentiating input driver circuit or at least an input driver circuit that approximates differentiation, for operating the input coil rather than allowing the original current signal carrying the information to be transferred to circulate therethrough. Such a differentiating driver circuit, or differentiating-like driving circuit, generates either a set or reset pulse in the input coil whenever the digital input signal transitions between low and high logic state current levels, or vice versa, and so in the magnetic field generated thereby. The inherent memory characteristics of certain “giant magnetoresistive effect” based sensors (having a hysteretic response characteristic between device saturation values), or the use of latching electronics in the receiver circuits connected to other kinds of “giant magnetoresistive effect” sensors having a more or less linear response between device saturation values (or even to such sensors with inherent memory characteristics), maintains the output of the signal isolator in its most recent logic state resulting from the coupled input signal until the receiver circuit detects a change of logic state in the coupled input signal, i.e. another set or reset pulse. High common mode rejection capabilities are typically designed into such signal isolators to ensure that the output responds only to such logic state changes in the input circuitry during normal operation.
However, under abnormal power supply performance conditions, or during circuit operation initiation after first switching on the digital isolator for a new next use session, there exists the possibility for the signal isolator output to exhibit an incorrect logic state. An incorrect logic state can result in erroneous data, damaged equipment, or can be a safety hazard when the device is used in man-machine interface equipment. Also, since the signal isolator operates on the rising and falling edges of the input signal transitions between logic states, i.e. is an edge triggered device, it is susceptible to spurious edge triggers leading to the risk of the output data being erroneously shifted in logic states so that it no longer tracks the logic states of the input signal. Spurious edge triggerings of the isolator can be caused by ground transients, electromagnetic interference or unexpected transients through the power supply and the like. This risk is especially high in low data rate input signals or low rate of change between logic states in the input signal where the time between desired logic state transitions, or in completing a transition, becomes relatively long thereby increasing the time for such unwanted occurrences.
Signal isolators having the receiver side thereof default to a predetermined one of the receiver output logic states is a desirable result to provide known output logic state whenever there is an occurrence of one of these failure conditions on the isolator input side to permit convenient implementation of safety response features in systems in which these signal isolators are used. However, the use of hysteretic response current sensors makes achieving this result difficult because there must be sufficient energy remaining present on the input aide to force the receiver to the desired output logic state through forcing a ferromagnetic material layer in the sensor to saturate if the receiver output is in the wrong logic state at the time of an input side failure. Such energy may not be available for some kinds of isolator input side failures as considerable values of input current in the input loop are required to reach such saturation values.
Avoiding this circumstance requires use of linear response current sensors which can be operated with input loop currents of smaller values since saturation of a ferromagnetic material layer therein is not needed. However, the current sensors must be operated with a bias current therein to be at a desirable operating point on the resistance versus input magnetic field (or input current) characteristic while being operated in their magnetic field sensing configuration in the presence of such smaller value inputs that lead to greater sensitivity to receiver circuit error sources. Thus, there is a desire to provide an arrangement that results in a predetermined receiver output logic state in the presence of failures on the input coil side of the isolation barrier therein in using linear response current sensors therein while operating such sensors on the receiver side of the isolation barrier in the presence of circuit error sources.